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The most prominent physicist who spent some time in Prague was

In **April 1911** Einstein was appointed
as a full **professor of theoretical physics**
at the German part of Prague's Charles University. By that time, he had
already won acclaim as the author of his special theory of relativity and
a number of successful studies in thermodynamics and molecular physics
and in particular in quantum theory and statistical physics.

Einstein moved from Zurich to Prague **together
with his first wife**, Mileva, and their two young sons, Hans Albert
and Eduard. The Einsteins **lived
in quarter Smichov** which was not considered the smartest
part of the city, but their flat (now in Lesnicka Street No. 7) was modern
(in contrast with Zurich there was already electricity installed) and Einstein
liked to recall his walks to the **Physical Institute
in Vinicna Street** over the Vltava river. Also, in Einstein's
words, the Institute was "excellent with a beautiful library".

Although Eisntein did not establish a very close rapport with Prague,
**he admired the romantic historical city**.
He associated himself with a **group of Jewish intellectuals**
who gathered in the evenings at Berta Fanta's home where philospohy was
discussed and music played. Here he met **Hugo Bergmann**, **Max Brod**
and **Franz Kafka**. While in Prague, Einstein was invited to
the first legendary **Solvay congress**
where he met Planck, Lorentz, Madame Curie, Poincaré, most of them for
the first time. Scientists also came to Prague to visit Einstein. One of
the most important was **Paul Ehrenfest**.

As a full professor, Einstein gave **regular
lectures** on mechanics, molecular physics, and thermodynamics.
In 1911 his lectures were held in the Clementinum, in 1912 in the building
of the present Faculty of Natural Sciences in Vinicna Street.

While in Prague, Einstein's interest in quantum theory diminished and
his **systematic work on a relativistic theory of
gravitation began**. **In Prague he published
eleven papers, six of them concerned with the theory of relativity**
(the non-relativistic works are devoted to the theory of specific heats
and interaction of radiation with matter). In fact, his papers published
during and shortly after his Prague period are particularly significant
in that they paved the way to his general theory of relativity.

This **effort was briefly summarized by Einstein
himself** in his foreword to the Czech edition of 1923 of his
famous little popular book "About the Special and General Theory of
Relativity in Plain Terms":

*"I am pleased that this small book ... should now appear
in the native language of the country in which I found the necessary concentration
for developing the basic idea of the general theory of relativity which
I had already conceived in 1908. In the quiet rooms of the Institute of
Theoretical Physics of Prague's German University in Vinicna Street, I
discovered that the principle of equivalence implies the deflection of
light rays near the Sun by an observable amount ... In Prague I also discovered
the shift of spectral lines towards the red ... However, the decisive idea
of the analogy between the mathematical formulation of the theory and the
Gaussian theory of surfaces came to me only in 1912 after my return to
Zurich, without being aware at that time of the work of Riemann, Ricci,
and Levi-Civita. This was first brought to my attention by my friend Grossmann
..."*

The foreword clearly emphasises two important effects which Einstein
discovered in Prague: the **deflection
of light** and the **gravitational
redshift** (in fact, the redshift had already been discussed in
Einstein's paper of 1907, in the foreward wrongly dated as 1908). The first
effect was described in the famous paper of 1911 "On the influence
of Gravity on the Propagation of Light". However, the paper is of
even greater importance: it contains what Eddington considered to be the
original statement of the **principle of equivalence**,
one of the most remarkable ideas in the history of science.

Also, the foreword does not mention Einstein's remarkable advance in
understanding the basic ideas and features of a complete relativistic theory
of gravity. These can be found in his other relativity papers from Prague.
They include the **equations of motion** of
a particle in a given gravitational field (derived from a variational principle),
the **influence of a given gravitational field on
other physical systems** (such as the electromagnetic field),
or the **non-linear character of field equations**
which describe the gravitational field due to a given source. All these
necessary ingredients of a relativistic gravitation theory Einstein arrived
at during his Prague stay.

However, during this period Einstein still assumed that the gravitational field can be described by a single function. This assumption lead to insurmountable difficulties and prevented him achieving the final formulation of the general theory of relativity during his time in Prague.

**Albert Einstein
left the city of Prague after his sixteen-month long stay in July 1912**
when he accepted the chair of theoretical physics at the Polytechnical
Institute of Zurich. Direct evidence suggests that Einstein was happy during
his professorship in Prague.

` `web page J.Podolsky, 30 Dec 1997,